116
Chapter 8 Some Cu(II) Binuclear Transition-Metal Complexes
have ferromagnetic ground-states, if each of the structures (15)-(18) participates in
resonance for both the S = 0 and S = 1 spin states. However,
1
covalent
can interact
with
1
ionic
, and some stabilization of
1
covalent
may then occur
i
.
Thus, according to this 6-electron 3-centre analysis for Cu(II)-X-Cu(II) complexes, there is a competition between an tendency for ferromagnetism due to a
preference for parallel spins in valence-bond structures of type 15, and a tendency
for antiferromagnetism when the S = 0 spin “covalent” structures of types 17 and
20 participate in resonance with S = 0 spin “ionic” structures of types 19 and 21.
Whichever has the greater tendency for a particular complex will determine the
magnetic properties of the ground-state.
For angular Cu-X-Cu linkages, the simplest type of Goodenough-Kanamori
theory involves resonance between “covalent” structures of types (16) and (17).
Calculations of this type (together with the mirror-image for structure (17),
namely (20)) have been reported by Barraclough and Brookes
30 .
The 6-electron 3-centre bonding scheme for Cu(II)-X-Cu(II) linkages with X =
halide or O
2– , is also appropriate for X = OH
– if it is assumed that two equivalent
p-orbitals of OH
– accommodate the lone-pairs of electrons. The O-H σ-bond of
HO
– must then utilize the oxygen 2s orbital for bonding. The oxygen (O
– ) valencestate now involves the promoted
6
sp , 1
V configuration. If it is considered that the
non-promoted
2 2
s p , 1
V configuration is the primary valence-state configuration
(as it would be in free OH
– ), then the primary oxygen orbital involved in the 3centre bonding is the 2p orbital
of Figure 8-5. The 4-electron 3-centre bonding
theory then becomes relevant for the Cu(II)-OH-Cu(II) linkages.
For both types of Cu(II) complexes of this chapter, we have not given
consideration to the utilization of the copper 4s and 4p orbitals for bonding to the
oxygen atoms of the carboxylate, hydroxo and chloride ligands. If these orbitals
are included, Pauling “3-electron bond” theory is still appropriate for the valencebond descriptions of the bonding. Geometrical requirements would require the
utilization of (approximately)
2
dsp hybrid orbitals of Cu
2+
. Prior to bonding to the
ligands, three of these orbitals are vacant, and one is singly-occupied. Because
each Cu
2+ ion of either complex is involved in bonding to four oxygen or halide
ligand atoms (see Fig. 8-1), it can participate in the formation of three electronpair bonds and one Pauling “3-electron bond”, as shown in valence-bond structure
(22), for example. The Cu(II) carboxylate and Cu(II) hydroxo or chloro dimers
are, therefore, examples of hypoligated complexes (Section 5-1). In the discussion
of this chapter, we have omitted the 4s and 4p orbitals, because the odd-electron
charge in a copper orbital is considered to be primarily 3d in character.
i This must include at least the overlap between the oxygen and copper atomic orbitals that are
singly-occupied in structures of types (17) and (20). This overlap is non-zero when the Cu-XCu bond-angle is not equal to 90°. For Cu(II)-Cl-Cu(II) linkages, this overlap is the only type
between the Cu(II)-Cl moieties that can be non-negligible in magnitude; the Cu-Cu distances,
which are greater than 3.2 Å are too large for the copper orbitals of Figure 8-5 to overlap
significantly.
Chapter 8 Some Cu(II) Binuclear Transition-Metal Complexes
have ferromagnetic ground-states, if each of the structures (15)-(18) participates in
resonance for both the S = 0 and S = 1 spin states. However,
1
covalent
can interact
with
1
ionic
, and some stabilization of
1
covalent
may then occur
i
.
Thus, according to this 6-electron 3-centre analysis for Cu(II)-X-Cu(II) complexes, there is a competition between an tendency for ferromagnetism due to a
preference for parallel spins in valence-bond structures of type 15, and a tendency
for antiferromagnetism when the S = 0 spin “covalent” structures of types 17 and
20 participate in resonance with S = 0 spin “ionic” structures of types 19 and 21.
Whichever has the greater tendency for a particular complex will determine the
magnetic properties of the ground-state.
For angular Cu-X-Cu linkages, the simplest type of Goodenough-Kanamori
theory involves resonance between “covalent” structures of types (16) and (17).
Calculations of this type (together with the mirror-image for structure (17),
namely (20)) have been reported by Barraclough and Brookes
30 .
The 6-electron 3-centre bonding scheme for Cu(II)-X-Cu(II) linkages with X =
halide or O
2– , is also appropriate for X = OH
– if it is assumed that two equivalent
p-orbitals of OH
– accommodate the lone-pairs of electrons. The O-H σ-bond of
HO
– must then utilize the oxygen 2s orbital for bonding. The oxygen (O
– ) valencestate now involves the promoted
6
sp , 1
V configuration. If it is considered that the
non-promoted
2 2
s p , 1
V configuration is the primary valence-state configuration
(as it would be in free OH
– ), then the primary oxygen orbital involved in the 3centre bonding is the 2p orbital
of Figure 8-5. The 4-electron 3-centre bonding
theory then becomes relevant for the Cu(II)-OH-Cu(II) linkages.
For both types of Cu(II) complexes of this chapter, we have not given
consideration to the utilization of the copper 4s and 4p orbitals for bonding to the
oxygen atoms of the carboxylate, hydroxo and chloride ligands. If these orbitals
are included, Pauling “3-electron bond” theory is still appropriate for the valencebond descriptions of the bonding. Geometrical requirements would require the
utilization of (approximately)
2
dsp hybrid orbitals of Cu
2+
. Prior to bonding to the
ligands, three of these orbitals are vacant, and one is singly-occupied. Because
each Cu
2+ ion of either complex is involved in bonding to four oxygen or halide
ligand atoms (see Fig. 8-1), it can participate in the formation of three electronpair bonds and one Pauling “3-electron bond”, as shown in valence-bond structure
(22), for example. The Cu(II) carboxylate and Cu(II) hydroxo or chloro dimers
are, therefore, examples of hypoligated complexes (Section 5-1). In the discussion
of this chapter, we have omitted the 4s and 4p orbitals, because the odd-electron
charge in a copper orbital is considered to be primarily 3d in character.
i This must include at least the overlap between the oxygen and copper atomic orbitals that are
singly-occupied in structures of types (17) and (20). This overlap is non-zero when the Cu-XCu bond-angle is not equal to 90°. For Cu(II)-Cl-Cu(II) linkages, this overlap is the only type
between the Cu(II)-Cl moieties that can be non-negligible in magnitude; the Cu-Cu distances,
which are greater than 3.2 Å are too large for the copper orbitals of Figure 8-5 to overlap
significantly.